Chapter 4 OBJECTIVES: * The outcomes of various interactions of x-rays with matter. * Inverse square law. * Primary and secondary exposure. * Protective shielding. By the end of this section, the student should be able to: 1. List the possible interactions x-rays may have with matter. 2. Recognize what the inverse square law is and how it applies to x-ray examinations. 3. Describe primary and secondary shielding. 4. Select shielding devices used for patient and employee radiation protection. 5. Describe important safety measures used by x-ray technologists to minimize exposure. Section 1 When x-rays interact with matter, one of three things will happen: 118 * Many of the incoming x-rays are completely absorbed in the object (photoelectric effect). * Some pass through (penetrate) the object and continue on their path. * Some interact with the object, causing a change in the strength of the incoming x-ray and its direction of travel (Compton scatter radiation). Scatter increases patient dose, degrades the image, and poses the greatest source of radiation exposure to the health care worker. Anything in the path of the x-ray beam may interact with and alter the course of the incoming x-rays including the patient, x-ray table, and film cassette. All are potential sources of scatter.
DISTANCE The most effective way to minimize exposure to a health care worker is to increase your distance from the source of the scatter.119 The Inverse Square Law states that the intensity of the x-ray beam is inversely proportional to the square of the distance of the object from the source.118,119 In other words, there is a rapid decrease in intensity as the beam spreads out over an increasingly larger area. For example, if a radiologic technologist takes one exposure with a source (x-ray tube) to object (the patient) distance of 40 inches, and then takes another exposure after doubling the distance, the intensity of the x-ray beam will only be ¼ of what it was at 40 inches. Conversely, if you are taking an exposure at 80 inches, and cut your distance in half to 40 inches, the intensity of the x-ray beam is now 4 times what it was before at 80".
Inverse Square Law: * Double the distance = ¼ the intensity of the x-ray beam. * Cut the distance in half = 4 times the intensity of the x-ray beam. The useful portion of the x-ray primary beam should never be directed towards a health care worker. If you must be the room while an exposure is being made, wear a lead apron.120 In a typical radiology department, 95% of a radiologic technologist's exposure comes from fluoroscopy and portable exams.120 Exposure cords on x-ray machines in an x-ray room are intentionally short to prevent the radiologic technologist from stepping into the room while the exposure is being made. On the other hand, cords on most portable x-ray machines are long so that the operator can stand as far away from the x-ray beam and source of scatter (patient) as possible. SHIELDING Shielding is the use of lead protective devices to prevent scatter radiation from exposing patients and health care workers.118-120 There are two types of shielding: primary and secondary. - Primary shielding reduces intensity of the primary x-ray beam after it passes through patient, film, and cassette. Lead lined walls, doors and lead glass prevent x-rays from traveling outside of the exam room. - Secondary shielding blocks scatter radiation. It does NOT provide adequate protection against primary x-ray beam.119,120 Secondary shielding is available in many forms. Lead is most commonly used because its physical properties and density make it so that x-rays cannot penetrate through it. To assure safety, always place shielding material between the x-ray tube and health care worker. The required safe thickness of a barrier is dependent on the distance between the source of radiation and the type of barrier. Common shielding includes contact shields (such as lead aprons and lead vinyl) and shadow shields. Lead apron: Lead impregnated vinyl aprons are worn anytime workers have to be in the x-ray room while an exposure is being made. They come in all sizes and weights. It is important to always hang lead aprons up when not in use and never drape them over counter-tops or chairs. If a tear or crease in the apron develops, have it x-rayed to see if it provides adequate protection. Any shield that is pitted, cracked, torn or damaged should be replaced.
Thyroid Shield Thyroid tissue is highly sensitive to radiation exposure and can be permanently damaged.121 Some cases of thyroid carcinoma have been linked to radiation exposure.120,121 Always try to pull the lead apron up around the thyroid for added protection. Use a thyroid shield if available whenever the thyroid is near the primary beam.
Gonad shield: these should be used for patients who are in their potentially reproductive years and are used whenever the gonads are near the primary beam. However, they are used only when the shield does not interfere with obtaining needed diagnostic information.
Portable lead shields on wheels can be placed between workers and the x-ray source or patient.
Lead vinyl sheets Lead gloves and glasses Sterile lead gloves can be worn during sterile procedures. Other Factors to Reduce Radiation Exposure Exposure time selected by the radiologic technologist is an important factor in reducing scatter. Always use the shortest time possible. This not only cuts down on scatter, but also reduces motion on the image. Scheduling of personnel should be done so that different employees take turns rotating through high dose situations, such as fluoroscopy rooms or portable exams. Length of procedures should be monitored. Positive beam limitation (pbl) reduces unnecessary patient exposure as well as scatter. PBL is a feature of the x-ray tube where collimation automatically closes the lead shutters of the collimator to match the size of the film being used. Only essential personnel should be allowed in the exam room during the procedure. Lead aprons are worn during any fluoroscopic procedure or whenever any non-patient remains in the room during the exposure. Portable lead barriers should be available in the room for lengthy procedures. Radiology personnel should not hold patients who are being x-rayed. Restraining devices should be used whenever possible. If someone needs to physically assist the patient during the exposure, non-radiology health care workers or family members of the patient should be asked to assist.
Those who must remain in the x-ray room during an exposure (fluoro, trauma, infant, elderly patient), must wear a lead apron. Stand behind the lead table drape which is attached to the fluoroscopic tower. If possible, stand behind someone else such as the radiologist. As shown in the drawing below, the head and foot ends of the table should be avoided because of the high scatter radiation levels present. Below: This drawing shows scatter radiation levels measured in millirem per hour (mR).
Key Points Remember from this Chapter - A lead vinyl protective curtain with no less than 0.25 mm of lead equivalent must be placed between the patient and the person performing and fluoroscopic procedure (tech or radiologist). - Total filtration (placed where x-ray photons exit the tube) must be 2.5 mm lead equivalent. This filtration removes weak, non-useful photons from the beam, which would have been absorbed in the patient's tissues, adding to their radiation exposure dose. - Primary radiation barriers protect from primary x-ray beam exposure. These include the doors, lead lined walls, and floors. Secondary radiation barriers are those that protect against scattered radiation (secondary). These include lead aprons, gloves, thyroid shields, and lead vinyl strips. If a primary x-ray beam is directed at an individual who is not the patient, secondary shielding will NOT offer adequate protection. - Increasing distance away from the source of radiation and wearing lead shielding are the two most important ways for health care workers to protect themselves from radiation exposure. - The minimum amount of protection that a lead apron must provide is 0.25 mm of lead equivalent. Some radiology departments chose to purchase heavier aprons with lead equivalent of 0.5 mm. Table of Contents |
What is the term for the reduction of intensity as it passes through material?
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